The present invention relates to a hydrogen storage alloy useful for a negative electrode of an alkaline storage battery and also to a method for production thereof.
A nickel-hydrogen storage battery has been recently noted as a new alkaline storage battery because of its high capacity, at least twice as high as that of nickel-cadmium batteries, and its environmental friendly nature. With the spread of portable instruments, this nickel-hydrogen storage battery is expected to further increase its performance.
A hydrogen storage alloy, when incorporated in a negative electrode of the nickel-hydrogen storage battery, generally undergoes spontaneous oxidation to form an oxide layer on its surface. Accordingly, a hydrogen storage alloy electrode fabricated from such a hydrogen storage alloy, when used as a negative electrode of the nickel-hydrogen storage battery, presents a problem of low initial battery capacity that is attributed to the initial low activity of hydrogen storage alloy.
A method has been recently proposed, for example, in Japanese Patent Laying-Open No. Hei 5-225975, which immerses a hydrogen storage alloy in an acid solution, such as a hydrochloric acid solution, to remove an oxide layer formed on its surface.
This method contemplates to remove an oxide layer from a surface of the hydrogen storage alloy by immersing it in the acid solution. However, nickel and cobalt hardly elute into the acid solution so that active sites as of metallic nickel (Ni) and cobalt (Co) appear on the hydrogen storage alloy surface.
Upon removal of the oxide layer by the above-described method, the active sites as of metallic nickel and cobalt appear on the hydrogen storage-alloy surface, so that the initial discharge capacity is increased. The reduction of electrical contact resistance between alloy particles also results to increase the high-rate discharge capacity to a slight degree. However, the electrical contact resistance between alloy particles is still too high to achieve a marked improvement in high-rate discharge capacity. Also, the method is still insufficient to prevent the buildup of pressure in the battery and improve a charge-discharge cycle life of the battery.
It is an object of the present invention to provide a hydrogen storage alloy which, when fabricated into an electrode for alkaline storage batteries, can provide an excellent charge-discharge cycle life performance, prevent the buildup of battery""s internal pressure during overcharge and improve high-rate discharge characteristics, and also to provide a method for production thereof.
The hydrogen storage alloy of the present invention, for use in alkaline storage batteries, has a CaCu5-type crystal structure and is represented by the compositional formula MmNixCoyMnzMl-z (wherein M represents at least one element selected from the group consisting of aluminum (Al) and copper (Cu); x is a nickel (Ni) stoichiometry and satisfies 3.0xe2x89xa6xxe2x89xa65.2; y is a cobalt (Co) stoichiometry and satisfies 0xe2x89xa6yxe2x89xa61.2; z is a manganese (Mn) stoichiometry and satisfies 0.1xe2x89xa6zxe2x89xa60.9; and the sum of x, y and z satisfies 4.4xe2x89xa6x+y+zxe2x89xa65.4). Characteristically, the hydrogen storage alloy has a bulk region surrounded by a surface region. The bulk region has a CaCu5-type crystal structure and a substantially uniform composition while the surface region has a graded composition. When the sum of percentages by number of cobalt (Co) atoms and copper (Cu) atoms present in the surface region is given by a and the sum of percentages by number of cobalt (Co) atoms and copper (Cu) atoms present in the bulk region is given by b, the relationship a/bxe2x89xa71.3 is satisfied.
In the present invention, the percentage by number of cobalt (Co) or copper (Cu) atoms present in the bulk or surface region may be referred to in terms of atomic %.
As stated above, in the hydrogen storage alloy of the present invention, the bulk region is a region that has a CaCu5-type crystal structure and a substantially uniform composition. The surface region is a region that surrounds the bulk region and has a graded composition. This surface region is the region of the hydrogen storage alloy particle that undergoes a change in composition when it is immersed in an acid treating solution according to the production method of the present invention which will be described later. By this immersion treatment, any oxides present on an alloy particle surface is removed while cobalt and copper are reductively deposited. As a result, the surface region is allowed to contain the increased amounts of cobalt and copper atoms compared to the bulk region. As described above, when the sum of percentages by number of cobalt atoms and copper atoms present in the surface region is given by a and the sum of percentages by number of cobalt atoms and copper atoms present in the bulk region is given by b, the relationship a/bxe2x89xa71.3 is satisfied. If a/b falls below 1.3, it may become difficult to obtain the effect of the present invention that improves a charge-discharge cycle life performance by lowering the contact resistance between hydrogen storage alloy particles to thereby increase a discharge capacity. It may also become hard to obtain the further effect of the present invention that not only prevents the build-up of battery""s internal pressure during overcharge but also improves high-rate discharge characteristics.
In the present invention, the region that encompasses a surface and its vicinity and has a graded composition is defined as the surface region, as contrary to the bulk region that has a substantially uniform composition. The surface region is generally observed to have a composition gradient such that the percentages by number of cobalt and copper atoms present therein increase toward the surface. Accordingly, the sum of percentages by number of those atoms present in the surface region, a, is determined by an average value in the surface region. Since the sum of percentages by number of those atoms measured at an intermediate depth of the surface region generally comes close to the average value in the surface region, the measured value may be taken as the sum of percentages by number of cobalt and copper atoms present in the surface region.
When the hydrogen storage alloy having the above-specified crystal structure and compositional formula is used for negative electrode material of an alkaline storage battery, the negative electrode shows the suppressed corrosion in an electrolyte to absorb the increased amount of hydrogen. This is the reason why the present invention uses the hydrogen storage alloy having such crystal structure and composition.
The surface region generally extends inwardly from an alloy particle surface to the depth of 80 nm.
The hydrogen storage alloy electrode of the present invention, for use in alkaline storage batteries, is obtained by loading the hydrogen storage alloy of the present invention in an electrically conductive substrate such as a punching metal.
The method for producing a hydrogen storage alloy of the present invention comprises a first step wherein alloy particles are prepared having a CaCu5-type crystal structure and represented by the compositional formula MmNixCoyMnzMl-z (wherein M represents at least one element selected from the group consisting of aluminum (Al) and copper (Cu); x is a nickel (Ni) stoichiometry and satisfies 3.0xe2x89xa6xxe2x89xa65.2; y is a cobalt (Co) stoichiometry and satisfies 0xe2x89xa6yxe2x89xa61.2; z is a manganese (Mn) stoichiometry and satisfies 0.1xe2x89xa6zxe2x89xa60.9; and the sum of x, y and z satisfies 4.4xe2x89xa6x+y+zxe2x89xa65.4), and a second step wherein the alloy particles are immersed in an acid treating solution containing a cobalt compound and a copper compound, each in the amount of 0.1-5.0% by weight based on the weight of alloy particles, to remove oxide layers on alloy particle surfaces and deposit cobalt and copper reductively so that surface regions are formed at alloy particle surfaces.
In the first step, alloy particles are prepared having the crystal structure and compositional formula as specified above. Those alloy particles are generally prepared by adding specific types of metals to a Misch metal consisting of a mixture of rare-earth metals. The technique used to prepare such alloy particles is not particularly specified. They may be prepared by casting the metal mixture into an ingot and then subdividing the ingot, or alternatively, by utilizing a gas atomizing or roll quenching technique. In view of sinterability of resulting alloy particles, the use of gas atomizing technique is preferred.
In the second step, the alloy particles are immersed in an acid treating solution containing a cobalt compound and a copper compound each in the amount of 0.1-5.0% by weight, based on the weight of alloy particles. This treatment not only removes oxide layers from alloy particle surfaces but also allows reductive deposition of cobalt and copper, resulting in the formation of surface regions at the alloy particle surfaces. Examples of acids useful for preparation of the acid treating solution include hydrochloric acid, nitric acid and phosphoric acid.
Examples of cobalt and copper compounds for addition to the acid treating solution include cobalt chloride (CoCl2), cobalt hydroxide (Co(OH)2), copper chloride (CuCl2) and copper hydroxide (Cu(OH)2).
The cobalt and copper compounds are incorporated in the acid treating solution in concentrations of 0.1-5.0% by weight, respectively; for the reasons which follow. The respective loadings thereof, if exceed 5.0% by weight, cause cobalt and copper to be deposited in excessively large amounts that result in the increased tendency of alloy particles to be oxidized and, if fall below 0.1% by weight, cause cobalt and copper to be deposited in small amounts that result in the difficulty to satisfy the relationship a/bxe2x89xa71.3. More preferably, the cobalt and copper compounds are incorporated in the acid treating solution in concentrations of 0.3-5.0% by weight, respectively.
Preferably, the acid treating solution is initially maintained at a pH in the range of 0.7-2.0. The pH of below 0.7 in some cases causes rapid oxidation of alloy particles that may dissolve even an interior of the hydrogen storage alloy. On the other hand, the pH of greater than 2.0 may result in the insufficient removal of oxide layers.
The acid treating solution may further contain at least one organic additive selected from the group consisting of 2,2xe2x80x2-bipyridyl, diethyldithio carbamate, 2-mercaptobenzothiazole and metanilic yellow. Such organic additives, if present, promote reductive deposition of cobalt and copper. The organic additive may preferably be incorporated in the amount of 5-50 ppm and contributes to further improvement of battery characteristics if kept within the specific range.
The method of producing a hydrogen storage alloy for use in alkaline storage batteries, in accordance with the present invention, is characterized as including the step of loading the hydrogen storage alloy of the present invention in an electrically conductive substrate such as a punching metal.